A microdisplay display system utilizes an array of spatial light modulators (the microdisplay) to modulate light in order to display images on a display plane. The microdisplay can modulate light by reflectively altering the transmission path of the light or blocking the transmission path of the light. Microdisplay display systems have enabled the production of high quality display systems at a reasonable cost.
One widely used form of microdisplay is a digital micromirror device (DMD). A DMD is made up of an array of hundreds of thousands of micromirrors that pivot along a common axis depending upon an image being displayed. A single image can be divided into a number of bit planes, which when displayed sequentially, are integrated by the human eye into a single image. There are multiple bit planes for each color of light in the microdisplay display system. An individual micromirror can assume one of two states, ON or OFF. When a micromirror is in an ON state, the micromirror can reflect light from a light source onto the display plane, while when the micromirror is in an OFF state, the light is reflected away from the display plane. The combined effect of all the micromirrors in the microdisplay, in conjunction with sequentially colored light, produces images on the display plane.
With reference now to FIGS. 1a and 1b, there are shown diagrams illustrating two commonly used forms of micromirrors. The diagram shown in FIG. 1a illustrates what is commonly referred to as a yokeless micromirror 100 and the diagram shown in FIG. 1b illustrates a yoked micromirror 150. The yokeless micromirror 100 includes a mirror 105 that is attached to a hinge 110, with the entirety resting on a hinge support structure 115. The mirror 105 pivots about the hinge 110 based on image data of the image being displayed. The yoked micromirror 150 also includes a mirror 155 that is attached to a yoke 160. The yoke 160 is attached to the hinge 165. Both the mirror 155 and the yoke 160 pivot about the hinge 165.
With reference now to FIGS. 2a through 2d, there are shown diagrams illustrating potential micromirror positions. When in normal operation, a micromirror 200 can be in one of two positions, a first position that corresponds to an ON position and a second position that corresponds to an OFF position. The diagram shown in FIG. 2a illustrates one of the two positions. When another bit plane is to be displayed, the micromirror can be commanded to move to a reset position. The diagram shown in FIG. 2b illustrates the reset position. Once in the reset position, the micromirror can move to either the ON position or the OFF position depending on the value of the image data. The diagrams shown in FIGS. 2c and 2d illustrate the ON position and the OFF position.
As the micromirror (mirror) 105 moves, a torque is applied to the hinge 110 in the same direction as the movement of the micromirror 105. If the micromirror 105 is operated in such a way that the micromirror 105 predominantly moves towards one side (one position, either ON or OFF), an effect known as hinge memory can occur. Hinge memory can be the result of the migration of the hinge material and if allowed to persist for an extended amount of time, can be a cause of a catastrophic failure of the micromirror 105. For example, in displaying an all black (or an all white) image, the micromirrors of the microdisplay will spend a vast majority of the time (greater than 95%) in a single position. Practical examples would include the superposition of a black box containing a close-caption text stream over the image in a television or an information display panel with text on a dark background.
The behavior of the micromirror 105 can be described by its duty cycle. The duty cycle describes the percentage of the time the micromirror 105 moves to a given position. For example, a 5/95 duty cycle means that 5% of the time, the micromirror 105 lands on a first position and 95% of the time, the micromirror 105 lands on a second position. As the duty cycle approaches 50/50, the micromirror 105 will have less hinge memory. For example, with a duty cycle of 50/50, the micromirror 105 generally will have no hinge memory (assuming that there was no initial hinge memory) since half of the time, the micromirror 105 is in the first position and half of the time, the micromirror 105 is in the second position.
The continued use of the microdisplay display system with a large disparity in the behavior (expressed as duty cycle) of the micromirror 105 over an extended period of time can result in a failure of the micromirror 105 by preventing the micromirror from changing position should a need to do so arise.
Similar problems exist in other display system technologies, such as degradation of phosphor screens in cathode ray tubes (CRT), erosion of electrodes in plasma screens, and degradation of polarizers in liquid crystal displays (LCD).